This invention relates to a process for the preparation of pyridines and/or hydrogenated pyridines that have been substituted with a hydrocarbon group at one or more carbon atoms, by the gas phase catalytic cyclization of .gamma.-cyanoketone in the presence of hydrogen.
It is known that these compounds can be prepared from a .gamma.-cyanoketone by processes described in British Pat. Nos. 1,304,155 and 1,425,698. These processes are carried out, for instance, by passing the .gamma.-cyanoketone, in a gas phase, and in the presence of hydrogen, over a catalyst containing a catalytically active metal or compound of metal selected from the group consisting of copper, silver, gold, iron, nickel, cobalt, ruthenium, rhodium, palladium, osmium, iridium and platinum. The resulting gaseous reaction mixture contains a fairly large quantity of hydrogen, which, after separation from the reaction mixture, can be recycled back to the reaction zone. However, it has been found that this separated hydrogen-containing gas, if directly recycled back to the reaction zone, causes the catalyst to lose activity and selectivity. In order to maintain the activity and selectivity of the catalyst used in the cyanoketone cyclization process at a high level, it has been suggested in U.S. Pat. No. 4,276,419 to first catalytically treat the hydrogen-containing gas by passing it over a catalyst containing iron, nickel, cobalt, or a compound of one of these metals, at a temperature of from about 300.degree. to 800.degree. C. before recycling it to the reaction zone.
It has now been found, however, that in such a catalytic treatment of the hydrogen-containing recycle gas, a carbon deposit will build up on the catalyst and the equipment used in the catalytic treatment in such a quantity that after a period of time, such as six days or so, the activity in the catalyst will be adversely affected, and the equipment will cease to function as required. The recycle of the hydrogen-containing gas must then be interrupted in order to replace the catalyst and clean the equipment, which can be quite costly.